Real-Time Electron Solvation Induced by Bursts of Laser-Accelerated Protons in Liquid Water

Understanding the mechanisms of proton energy deposition in matter and subsequent damage formation is fundamental to radiation science. Here we exploit the picosecond (${10}^{-12}\mathrm{s}$) resolution of laser-driven accelerators to track ultrafast solvation dynamics for electrons due to proton radiolysis in liquid water (${\mathrm{H}}_2\mathrm{O}$). Comparing these results with modeling that assumes initial conditions similar to those found in photolysis reveals that solvation time due to protons is extended by $>20\mathrm{ps}$. Supported by magnetohydrodynamic theory this indicates a highly dynamic phase in the immediate aftermath of the proton interaction that is not accounted for in current models.

Figure 1

Scheme of the experimental setting. The main pulse is focused on the target to produce the proton bunch via TNSA. The protons enter the cell trough a Teflon window. The chirped probe pulse propagates through the sample and is then imaged onto the entrance slit of the imaging spectrometer. Each optical streak is then obtained by dividing two back-to-back probe spectra; one with x rays and protons accelerated by the main laser (data shot) interacting in the ${\mathrm{H}}_2\mathrm{O}$ pixelwise divided by one without (probe only signal). Depth into the sample is defined in the proton propagation direction ($z$).

Figure 2

Observation of proton interaction with time and depth. (a) Optical streak of the samples transmission during x-ray and proton interaction in ${\mathrm{H}}_2\mathrm{O}$ (Sig). The maximum incident proton energy was $12.3\pm 0.2\mathrm{MeV}$. It should be noted that higher energies were available but given that here we are primarily interested in the stopping dynamics for protons interacting in ${\mathrm{H}}_2\mathrm{O}$ energies were chosen to allow this to be studied unambiguously. The interaction of higher energies will be the subject of a future publication. (b) Numerical gradient of (a) ($\mathrm{dSig}/\mathrm{dt}$). Here the spatiotemporal distribution of the solvation is revealed. Especially the x-ray signal is observed to stop more than 200 ps prior to protons’ arrival. (c) Computational result of secondary electron counts calculated from reproduction of the experimental proton bunch [16, 21] (Mod). In each case, the origin of the time axis corresponds to the point of the x rays and protons’ emission at the target. Experiments were also performed for which no drop in transmission below noise was observed due to the x-ray burst. Under these conditions the signal due to protons was identical to within uncertainty as Figs. 2 and 2.

Figure 3

Opacity caused by solvated electrons. Shown is the mean signal of Fig. 2 at $500\pm 50\mu \mathrm{m}$ benchmarked against the transmission caused by the calculated solvation yield. The computation was done for the proton bunch given in Fig. 2. Both lineouts were normalized to 1. The delay of the experimental signal drop with respect to the simulation can be seen by $\mathrm{\Delta }{t}_{\exp }-\mathrm{\Delta }{t}_{\mathrm{sim}}=22\pm 1\mathrm{ps}$.